A screen printing stencil can be made in two or a combination of the two ways. An indirect screen printing stencil is prepared by utilising a presensitised photostencil material which has been coated onto a temporary film support. The presensitised film is exposed through a photographic positive, developed to produce an image on that support, which is then transferred to the screen printing mesh to provide a printing mask.
A direct screen printing stencil is prepared by applying a photosensitive solution, emulsion or dispersion to the surface of the screen printing mesh, drying, exposing through a photographic positive and washing out the unexposed areas of the stencil again to provide a printing mask.
The direct process is said to provide superior wear properties and the indirect process to give superior printing with less saw tooth effect caused by shrinkage of the drying coating on the mesh. Also known is the so called direct/indirect process in which the indirect film is laminated to the mesh by a direct stencil solution, emulsion or dispersion, exposed through a photographic positive and the unhardened areas developed away to provide a printing mask.
The present invention finds particular application in direct photostencil coatings and their use in screen printing.
Useful compositions for direct screen printing coatings are described in GB-A-998838 and GB-A-1069366. Here a support is coated with a colloid layer containing an aldehyde in admixture with a diazonium compound or with a condensation product of formaldehyde and a diazonium compound. The diazonium compound is derived from certain di-amino compounds and it may be modified to form a triazene derivative using certain secondary amines.
The sensitised colloid layer is applied to a tensioned mesh or porous substrate and in the direct screen printing process this would usually be achieved with the aid of a suitable applicator known to those skilled in the art of making direct screen printing stencils. Multilayers of the coating may be applied, sometimes with intermediate drying, in order to build up a suitably thick layer to give the coating strength and to minimise the saw tooth effect caused by the mesh. The mesh support is usually polyamide or polyester, metal or silk. The applied coating is dried with warm air to provide a film. This coating is then exposed through a photographic positive in a vacuum frame to ultraviolet radiation. The unexposed areas are washed by water from the mesh, the remaining exposed areas forming a printing mask which, when dried, is suitable for the screen printing process.
The diazo sensitised colloid described above may be of any number of types as described in GB-A-998838 but are more normally of polyvinylalcohol produced by the saponification of polyvinylacetate. Fillers such as finely dispersed polyvinyl acetate may be added to the coating to improve the properties of the stencil in some way for a particular application. For example in a coating using an aqueous dispersion of polyvinyl acetate as filler the ratio of filler to colloid can influence the resistance of the resultant screen printing stencil to the various screen printing media. Thus a coating for producing a stencil for solvent based printing media would only contain a relatively small amount of polyvinyl acetate filler as this could be affected by the solvent. On the other hand a stencil for water based printing media would have a larger amount of polyvinyl acetate filler to imporve the water resistance of the coating. The amount of polyvinyl acetate would be limited by the amount that could be included in the formulation but still allow development by a water spray to give good image reproduction.
It is already known to those skilled in the art of producing stencils that the treatment of the above mentioned stencils with strongly acidic solutions after development does render them more resistant to water. However, this process in fact gives only a marginal improvement. Water resistance may however be further improved by the incorporation of acid crosslinkable resins dispersed into a water soluble colloid, preferably polyvinyl alcohol. Hitherto this has been accomplished by dissolving a water insoluble reactive resin such as a urea formaldehyde resin in a solvent and adding this with vigorous stirring to the colloidal solution. When sensitised and used to form a screen printing stencil, such a stencil can be hardened after exposure and development by treatment with an acidic solution, which causes the acid reactive resin to crosslink. This type of emulsion has the disadvantage that the solvent used escapes during stencil drying giving a strong unpleasant odour and a fire hazard. It is also possible to mill certain reactive copolymers into a screen coating solution but these provide only a weak crosslinked film. Water soluble acid crosslinkable copolymers when added to the coatings have been found to be unstable or provide only limited water resistance when cured.
It is an object of this invention to benefit on the one hand from the inclusion of acid crosslinkable dispersible resins in the composition but without incurring the penalties either of using dangerous or noxious solvents or of losing composition stability.
Published Japanese Applications Nos. 137833/83 and 137834/83 disclose a photosensitive composition comprising an aqueous solution of a saponified copolymer of an N-alkoxymethyl acrylamide and vinyl acetate. When exposed to actinic radiation a photoinitiator also included in the composition initiates crosslinking of the alkoxymethyl groups so that the composition acquires a useful degree of water resistance and thus could be used in a screen printing application.
However there are a number of disadvantages with these compositions, at least insofar as they are applicable to screen printing.
Firstly they are compositions designed for use in photoengraving and the coating of television tubes and would be unlikely to provide the wear characteristics to enable coated films to undergo the rigours of screen printing with water based inks, without post image development processes that would be incompatible with the other materials used in the screen printing process.
Further, the crosslinking N-methylol ether groups are attached to the water soluble polyvinyl alcohol and are thus likely to be affected by water soluble acid produced by degradation of water soluble diazo sensitisers, whether this be thermal or photolytic. Thus compositions with N-methylol ether groups in the aqueous phase might be expected to show signs of instability.
Thus it is a further object of the present invention to overcome or at least mitigate these problems.